Master of Science (MS), Ohio University, 2017, Mechanical Engineering (Engineering and Technology)

Evaporation of water in hydrophobic confinement has been a subject of numerous studies because of its key role in functioning and self-assembly of many biologically relevant systems, such as protein folding, formation of lipid bilayers, operation of ion channels, etc. Evaporation is an activated process and hence is a rare event in molecular time-scales. There is no consensus on a continuum thermodynamic theory which captures different aspects of the process satisfactorily. We study the simple case of evaporation of water confined between two rigid hydrophobic walls of tunable hydrophobicity, and adopt nucleation theory as our continuum thermodynamic approximation. We propose analytical expressions for free energy barrier and size of the critical vapor tube necessary for evaporation to occur. In the theory, we incorporate the effect of line tension, that has been neglected so far. To validate the expressions and explore role of line tension, we obtain free energy barriers and critical radii to evaporation from molecular simulations, by employing Indirect Umbrella Sampling (INDUS) and committor probability analysis methods. By comparing the results from simulations and theory, we find that the role of line tension is crucial in evaporation of water in hydrophobic confinement.

Committee: Sumit Sharma Dr. (Advisor); Horacio Castillo Dr. (Committee Member); Sarah Hormozi Dr. (Committee Member); Keerti Kappagantula Dr. (Committee Member) Subjects: Chemical Engineering

MS, Kent State University, 2016, College of Arts and Sciences / Department of Physics

This thesis considers thin films of 4'-n-octyl-4-cyano-biphenyl (8CB). At room temperature, 8CB forms a monolayer on a water surface in open air, given the right ratio of area to number of molecules. Then, as that ratio is decreased by a small amount, either by decreasing the area of the surface or by adding more 8CB, regions that are three molecules thick appear and coexist with an overall background that is one molecule thick. The separation line between the two distinct fluid phases, monolayer and trilayer, has a line tension, or energy per unit length. As more 8CB is added, trilayer predominates and becomes the background, with monolayer islands, but the behavior of the line separating the two phases remains the same. A Brewster angle microscope images 8CB surface layers on water with good contrast between phases. The objective of this thesis is to measure the line tension associated with this phase boundary as a function of temperature, with particular attention to temperatures at which the bulk liquid crystal would not form layers. A constant non-zero line tension means that domains in a perturbed region of the surface will relax to a circular shape if allowed to reach equilibrium. A hydrodynamic model is employed to correlate the changing shape of a domain during the relaxation process with an associated line tension.

Committee: Elizabeth Mann PhD (Committee Chair); Hamza Balci PhD (Committee Member); John Portman PhD (Committee Member) Subjects: Physics

PHD, Kent State University, 2013, College of Arts and Sciences / Department of Physics

A Langmuir film, which is a molecularly thin insoluble film on a liquid substrate, is one practical realization of a quasi-two dimensional matter. The major advantages of this system for the study of phase separation and phase co-existence are (a) it allows accurate control of the components and molecular area of the film and (b) it can be studied by various methods that require very flat films. Phase separation in molecularly thin films plays an important role in a range of systems from biomembranes to biosensors. For example, phase-separated lipid nano-domains in biomembranes are thought to play crucial roles in membrane function. I use Brewster Angel Microscopy (BAM) coupled with Fluorescence Microscopy (FM) and static Light Scattering Microscopy (LSM) to image phases and patterns within Langmuir films. The three microscopic techniques – BAM, FM and LSM - are complimentary to each other, providing distinct sets of information. They allow direct comparison with literature results in lipid systems. I have quantitatively validated the use of detailed hydrodynamic simulations to determine line tension in monolayers. Line tension decreases as temperature rises. This decrease gives us information on the entropy associated with the line, and thus about line structure. I carefully consider the thermodynamics of line energy and entropy to make this connection. In the longer run, LSM will be exploited to give us further information about line structure. I have also extended the technique by testing it on domains within the curved surface of a bilayer vesicle. I also note that in the same way that the presence of surface-active agents, known as surfactants, affects surface energy, the addiction of line active agents alters the inter-phase line energy. Thus my results set to stage to systematically study the influence of line active agents -`linactants’ - on the inter-phase line energy. Hierarchal self-assembled chiral patterns were observed as a fun (open full item for complete abstract)

Committee: Elizabeth Mann Dr. (Committee Chair); David Allender Dr. (Committee Member); Hamza Balci Dr. (Committee Member); J. Mann Dr. (Committee Member); Hiroshi Yokoyama Dr. (Committee Member); Qi-Huo Wei Dr. (Committee Member) Subjects: Biophysics; Physics

PHD, Kent State University, 2007, College of Arts and Sciences / Department of Physics

A Langmuir film is a well-controllable structure defined as a molecularly thin layer at the gas-fluid interface. This film forms quasi-two-dimensional analogues of gas, liquid, liquid-crystal, solid, and yet other phases if multilayers are possible. Any two of these phases may coexist, with energy associated with the boundary. This energy per unit length, defined as line tension, determines the shape and dynamics of coexisting phases, with possible applications to the multi-component biological cell membranes. A good understanding of this line tension will lead to a better idea of what controls the membrane configuration. In this work, I use Brewster angle microscopy, the Langmuir trough, the Wilhelmy plate technique, the four-roll mill, and other techniques to study the behaviors of different coexistence systems, including polymers and liquid crystals, in Langmuir films at the air-water interface. I use the dynamic response of the monolayers to explore line tension. Previous theories connecting dynamics and line tension in Langmuir films are limited and hard to access experimentally. I have collaborated with a group of mathematicians, physicists, and engineers on developing a hydrodynamic theory considering the coupling between the bulk subfluid phase and the surface phases, which gives a boundary-integral formulation that can be efficiently treated both analytically and numerically. Two cases were considered: hole-closing and domain relaxation. With modifications on some practical experimental techniques to reach the goal of this work, I directly compare these experiments to both analytic and numerical results, in order to both confirm the results and help to develop the new theory. The case of hole-closing in a polymer monolayer directly confronts the usual approximation of strictly horizontal flow. Line tension analyzed with new theory allows a factor of ten improvement in the accuracy and precision of line tension measurements. In the specific case of the coexi (open full item for complete abstract)

Committee: Elizabeth Mann (Advisor) Subjects: